The invention relates to ultrasonic thickness gauges and particularly to a transducer construction and gauge calibration method that provides for self-calibration of the zero time point of the gauge without a test block.
Ultrasonic measurements of thickness are most commonly made by timing the interval for an ultrasonic wave pulse to traverse a sheet or plate, and converting the measured time to thickness. Some ultrasonic thickness gauges such as those designed for corrosion work utilize so-called "dual element", or "pitch-and-catch", transducers. The ultrasonic wave pulse is transmitted by a "pitch" transducer, mounted on a block of material, through the material to the piece to be measured. The wave pulse is reflected from the back side of the piece and received by a "catch" transducer, similarly mounted on a block of material.
This dual transducer design is used because it provides greater thin-range resolution and signal-to-noise ratio on the rough surface produced by corrosion. The blocks of material on which the transducers are mounted provide a barrier between the transducer elements and the piece to which the transducer is applied, and add a time delay to the time interval required for the wave to traverse the material being measured. Each transducer element is mounted on a different block of delay material, and the blocks are separated by an acoustic barrier to prevent ultrasonic waves passing directly from one block to the other.
Because the blocks of delay material add a time delay in the passage of the interrogating wave pulse signal, the zero time point for the time interval measurement, from which the thickness of the piece is derived, is not coincident with the transmission of the signal, but instead occurs at some time a few microseconds later. The exact zero time point is determined by the length and sound velocity of the delay material and the geometry of the transducer assembly. A zero point calibration adjustment is provided on the instrument to electronically set the zero time point.
For ultrasonic thickness gauging instruments of this type presently available, the zero point calibration is accomplished by coupling the transducer to a test block of known thickness and adjusting the instrument until the known correct thickness of the test block is displayed. However, the zero point of the transducer can change due to variations in temperature or due to wear of the blocks of delay material. In particular, if transducer utilizing plastic delay materials are used to make measurements on hot surfaces, the zero point calibration must be frequently checked in order to compensate for the change in sound velocity in the delay material as it heats up. The frequent use of a test block to reset the zero point is undesirable. It requires time, and care, and both hands interrupting the measurement procedure, and requires the availability of the test block.
Accordingly, it is a principal object of the invention to provide an apparatus and method for the zero point calibration of an ultrasonic thickness gauge without requiring use of a test block.
Another principal object of the invention is to provide a means of self-calibration of an ultrasonic thickness gauge that automatically compensates for changes of temperature of delay material or changes in length due to wear of the delay material blocks.
Still another object of the invention is to provide a construction and method for such a self-calibration that is simple in design, reliable and accurate in use, convenient to operate and inexpensive to produce.